Distribution and palaeoecological indicator value of diatoms in Holocene coastal deposits of the southern North Sea (Lower Saxony, Germany)

The development of the North Sea coastal landscapes during the past 8000 years has been strongly influenced by sea-level fluctuations that involved cyclical transgression, stagnation and even regression phases. Transgressive phases are reflected in N W Germany by Holocene coastal deposits con­ sisting mainly of sand, silt and clay, whereas phases of regressive coastal development are indicated by the presence of peat layers. Although the East Friesian barrier islands in their present state are not older than 2000 years, they possess material that is amenable to palaeoecological investigations. Fossil salt marsh and tidal flat horizons provide natural sea-level indicators that record fluctuations of the mean high tide level in the southern North Sea. This paper presents the results of diatom analyses from boreholes in coastal regions and on the East Friesian barrier islands. Diatom assemblages of the clastic units of marine transgression phases are generally characterized by marine pelagic and littoral diatoms. Nevertheless, oligoand mesohalobous species also occur, predominantly in direct contact with the basal peat layer suggesting that the first marine transgres­ sion proceeded slowly. Tidal flat deposits are charac­ terized by polyand mesohalobous species, whereas 'Anschriften der Verfasser: PD Dr. HOLGER FREUND, Dr. G. GERDES, Dr. F. WATERMANN, Institut für Chemie und Biologie des Meeres (ICBM), Meeresstation, Schleusenstr. 1, D-26382 Wilhelmshaven; E-mail: holger.freund@icbm.terra mare.de lagoonal sediments are dominated by littoral brack­ ish and sometimes fragile diatoms indicating low sa­ linity and calm depositional conditions. Intercalated Phragmites fen peat layers show an increase in littoral oligohalobous species reflecting regressive phases of sea level. Salt-marsh deposits on the East Friesian is­ lands, as well as those in contact with the Pleistocene hinterland, mainly contain aerophilous, littoral spe­ cies. Flooding events and increasing marine influ­ ences are documented by higher amounts of pelagic marine diatoms. Dune slack deposits, which only occur on the islands, and deposits in the uppermost salt-marsh areas show a clear dominance of oligo­ halobous species suggesting that marine influences are restricted in those environments. [Verbreitung und paläoökologischer Indikator­ wert von Diatomeen in holozänen Küstenabla­ gerungen der südlichen Nordsee (Niedersach­ sen; Deutschland)] Kurzfassung: Die Entstehung der niedersächsi­ schen Küstenlandschaften ist während der letzten 8000 Jahre eng mit Meeresspiegelfluktuationen ver­ bunden, die Phasen von Meeresspiegelabsenkungen und -anstiegen ebenso umfassen wie Stillstandspha­ sen. Transgressive Abschnitte zeichnen sich vor allem durch sandige und tonig-schluffige Ablagerungen aus, während Regressionsphasen zum Teil durch Torf gekennzeichnet sind, der sich in Phasen ver­ langsamten Meeresspiegelanstieges seewärts auf ma­ rine Ablagerungen ausbreiten konnte. Auch auf den Ostfriesischen Inseln ergeben sich Möglichkeiten paläoökologischer Untersuchungen, die ungefähr bis in die Zeitenwende zurückreichen. Hier können vor Distribution and palaeoecological indicator value of diatoms 4 9 allem ältere Salzwiesenhorizonte und Wattschichten genutzt werden, die heute infolge von Strandver­ schiebungen am seeseitigen Strand einiger Inseln erosiv freigelegt werden. Diese Horizonte bilden natürliche Pegelmarken, an denen zurückliegende Meeresspiegelstände rekonstruiert werden können. In diesem Artikel werden diatomologische Ergeb­ nisse vorgestellt, die an Bohrungen und Profilen der festländischen Küste und Ostfriesischen Inseln erar­ beitet wurden. Die Diatomeen-Vergesellschaftungen klastischer Sedimente, die während Transgressionsphasen abgesetzt wurden, sind generell durch pelagische und litorale marine Arten charakterisiert. Im Kontakt zu den Ablagerungen des Basaltorfes treten verstärkt oligobis mesohalobe Arten hinzu. Dies legt den Schluss nahe, dass sich dieser erste transgressive Kontakt nicht erosiv, sondern fließend voll­ zogen hat. Die Wattablagerungen sind vornehmlich durch eine Artenkombination polybis mesohalober Arten gekennzeichnet, während lagunäre Sedimente häufiger litorale brackische und schwach verkieselte Arten aufweisen. Dies deutet auf einen geringeren Salzgehalt und ruhige Ablagerungsbedingungen hin. Die eingeschalteten Niedermoortorfe mit Phragmites zeigen einen Anstieg litoraler oligohalober Arten und bezeugen somit den Rückgang der Salinität und mithin eine regressive Phase innerhalb der Mee­ resspiegel-Entwicklung. Salzwiesenablagerungen sowohl auf den Inseln als auch im Kontakt zum pleistozänen Hinterland werden durch aerophile litorale Spezies charakterisiert, Uberflutungsereignisse deuten sich durch stärkere Nachweise pelagischer mariner Diatomeen an. Sowohl Ablagerungen feuchter Dünentäler, die im Untersuchungsgebiet nur auf den Ostfriesischen Inseln auftreten, als auch die am höchsten gelegenen Bereiche der Salzwiese beherbergen eine Vielzahl oligohalober Spezies und unterstreichen auf diese Weise den zurückgehenden marinen Einfluss.

Institut für Chemie und Biologie des Meeres (ICBM), Meeresstation, Schleusenstr. 1, D-26382 Wilhelmshaven; E-mail: holger.freund@icbm.terra mare.de lagoonal sediments are dominated by littoral brack ish and sometimes fragile diatoms indicating low sa linity and calm depositional conditions. Intercalated Phragmites fen peat layers show an increase in littoral oligohalobous species reflecting regressive phases of sea level. Salt-marsh deposits on the East Friesian is lands, as well as those in contact with the Pleistocene hinterland, mainly contain aerophilous, littoral spe cies. Flooding events and increasing marine influ ences are documented by higher amounts of pelagic marine diatoms. Dune slack deposits, which only occur on the islands, and deposits in the uppermost salt-marsh areas show a clear dominance of oligo halobous species suggesting that marine influences are restricted in those environments.

Introduction
Diatoms are photosynthetic single-celled algae with a Si0 2 -cell wall that generally fossilizes well. This opaline frustule is composed of two valves which fit together by girdle bands. The taxonomy of diatoms is mainly based on the shape and structure of these siliceous valves.
Most of the littoral pennate species are associat ed with specific substrates that mainly influence the distribution of these species. The following terms are used: epiphytic, growing attached to other plants; epipsammic, growing on sand, and epipelic, growing on mud.
Epiphytic diatoms, mostly associated with macroalgae, are characteristic of permanently sub merged low-energy environments such as ditch es, ponds, lakes and lagoons (Vos & DE WOLF 1988). In intertidal areas epipsammic diatoms are usually part of the littoral populations in lagoons (Vos & DE WOLF 1988), whereas on in tertidal mud flats and salt marshes, epipsammic forms are insignificant. Littoral epipelic species are dominant in these environments (ADMIRAAL 1984). However, epipelic diatom assemblages in salt marshes differ from those on mud flats, as salt-marsh species such as Diploneis interrupta are adapted to irregular inundation (DENYS 1994). The coastal zone is not only character ized by littoral, but also by a high abundance of predominantly planktonic pelagic species living in the water column of the North Sea.
The key difficulty in palaeoecological diatom research is to distinguish autochthonous spe cies from transported, allochthonous species. This differentiation is of major importance especially in tidal and estuarine environments (BROCKMANN 1940;SIMONSEN 1962;ZONG 1997;ZONG & HORTON 1999). In tidal en vironments such as tidal channels, the alloch thonous taphocoenosis occasionally outnumber the autochthonous taphocoenosis (SIMONSEN 1962 (Vos & DE WOLF 1988;DENYS 1994;FREUND & STREIF 1999, 2000, sea-level change and related trans-and regressive coastal developments (STABELL 1982;PALMER & CLAGUE 1986), and the reconstruction of palaeosalinity gradients (SIMONSEN 1962;MOORE and MCINTIRE 1977;JUGGINS 1992). Whereas earlier diatom investigations (BROCKMANN 1928(BROCKMANN , 1940GROHNE 1959;BENDA & STREIF 1971) from the NW German coastal area have been limited to qualitative studies, quantitative diatom investigations of Holocene deposits are rare in contrast to the Netherlands (ANDERSON & Vos 1992;Vos & de WOLF 1994). In this pa per, data on the relative abundance of diatoms in sedimentary sequences from NW Germany are used to characterize facies changes caused by sea-level fluctuations.

Quaternary sea-level fluctuations in the southern North Sea
During the Weichselian last glacial maximum, 22 000 to 18 000 years BP, the sea level was ca 110-130 m lower than today and so many parts of the North Sea were dry land (JELGERSMA 1979;CAMERON et al. 1987;LONG et al. 1988;FLEMMING & DAVIS 1994). Subsequently, sea level began to rise during the course of Holocene climatic amelioration. At 9000 years BP the northern limits of the Doggerbank were reached (ZAGWIJN 1986). At this time fresh water conditions still prevailed in the southern part of the North Sea basin, as documented by pollen analyses (BEHRE et al. 1985). The subse quent sea-level rise triggered the accumulation of coastal sediments in NW Germany (STREIF 1990;FLEMMING & DAVIS 1994). Between 8600 and 7100 BP sea-level continued to rise at a rate of about 2 m per 100 years (STREIF 1990), starting from 45 m NN and reaching 15 m NN in the latter phase of this period (NN = German zero datum; -a.s.l.). The pre-existing Pleistocene landscape was gradually inundated and covered with marine deposits, starting with the low-lying valley systems (Vos & DE WOLF 1993;FLEMMING & DAVIS 1994;LUDWIG et al. 1979 Holocene transgression was not continuous but interrupted by distinct cycles of marine stagna tion and even regression phases of sea level. This is evident from the presence of peat layers inter calated in the otherwise clastic Holocene coastal deposits. Due to the lack of dateable material, our knowledge of sea-level fluctuations over the last 2000 years is somewhat limited. New evi dence documenting distinct sea-level changes during that period is provided by studies of fossil salt-marsh and tidal flat horizons on the East Friesian Islands (FREUND & STREIF 1999, 2000. Increasing human activity has greatly influenced the development of the coastal landscape since Medieval times (ca 1000 years BP). The coastline was actively shaped through the building of dikes in the 11 th century, culmi nating in a continuous system of winter dikes during the 13 th century. This prevented natural sedimentation in the dike-protected hinterland. As a consequence, clayey coastal sediments be came compacted due to drainage, and resulted in a higher storm surge level. This led to severe breaches in dike structure and the formation of new tidal bays such as the Jade bay near Wil helmshaven or the Dollart near Emden. In con trast, the East Friesian Barrier islands retained a more or less natural/semi-natural environment until the construction of dikes commenced in Modern times.

Study sites
This work is based on the study of six sampling sites in the southern North Sea (Fig. 1 (MÜLLER 1977). The Schweiburg drill site, E of the Jade bay ( Fig. 1) is situated in what may be a tidal bay. The Wangerland drill site lies ap proximately 18 km NW of Wilhelmshaven in the Wangerland marshland (Core Wangerland 2 and 5; Fig. 1, Tab.l). This study area, now named Crildumer Bay, was a sheltered bay in former times but has since silted up. It was most likely subdivided by a peninsula (PET-ZELBERGER 1997). The cores presented in this paper are located in the southern section of the former bay. Palaeochronological investigations into former sea-level fluctuations, based on evidence from salt-marsh and tidal flat horizons of the East Friesian Islands, have so far been carried out on Memmert, Borkum, Juist, Langeoog, Wangerooge and Mellum. In this paper results from the study sites of Borkum, Juist and Lan geoog are presented (Fig. 1, Tab. 1). Conditions for palaeoecological and geologi cal studies are most favourable on the seaward side of Juist as palaeohorizons are particularly well exposed (Juist profiles 3, 6 and 18; Tab. 1, FREUND & STREIF 1999, 2000FREUND 20032004). These salt marshes were originally formed on the landward side of the island but were blanketed and preserved by dune sand following a north-south dislocation. Coastal erosion and shoreline displacement has lead to the exposure of these deposits in more recent times (Fig. 2). The Borkum profile (pro file "Am Hopp") is located in a recent salt-marsh area south of the town of Borkum (Tab. 1). The search for the most complete sequence of salt marsh and tidal flat deposits was carried out with a hand driven corer. On Langeoog the sampling site is situated near the centre of the village of Langeoog. During the geological survey of the 1960s two intercalated organic layers, most likely indicating variations in island development, were described by BARCKHAUSEN (1969). At present the availability of suitable coring sites is restricted as many promising areas have been built-over. Nevertheless, a small area of fallow land in the centre of the village has produced one organic horizon (profile 3 "Kiebitzweg", Tab. 1). The survey followed the method used on Borkum. On all islands sam ples were extracted from excavated holes using metal sampling boxes.

Material and Methods
Diatoms were separated according to SCHRÄD ER (1973). Counts were obtained using a Zeiss Axiophot microscope under phase contrast at Distribution and palaeoecological indicator value of diatoms 53 xlOOO magnification, thus providing data on relative abundance. Frequencies are expressed as percentages of the total number of diatom valves counted (TDV; minimum of 200 valves in each sample). Counts of samples of 0.1-lml volume (depending on the suspension) were made in triplicate using counting chambers (UTERMÖHL 1958) under an inverted microscope (x400 magnification). If no diatoms were found by applying the above method, the following pro cedure was used. Samples were homogenized and dried, treated with 15% hydrogen peroxide for one hour, suspended in water and sieved through 500pm and 63pm mesh. The resulting fractions were dried for 12h at 60°C. Diatoms were analysed in subsample triplicates of 500mg minimum. Those of the >63pm fraction were counted at x200 magnification using a Zeiss Stemi SV 11 binocular microscope. Diatoms were identified using DREBES (1974), HUSTEDT (1957) and PANKOW (1990). Nomenclature fol lows HARTLEY et al. (1996). The diatom species were divided into groups depending on salinity tolerance according to SIMONSEN (1962): (i) polyhalobous-marine taxa (-30%o), (ii) mesohalobous-brackish taxa (0.2-30%o), and (iii) oligohalobous taxa (0-20%o). Halophilous and indifferent taxa can tolerate brackish and freshwater conditions, but the former have their optimum in brack ish water while the latter are regarded as having their optimum in freshwater conditions. An extended classification is based on Vos & DE WOLF (1993), in which diatoms are classified in relation to their saline optimum and according to their specific sedimentary environments. Diatom counts are plotted using the TILIA graph program by GRIMM (1993

Results
Although all sampling sites are situated around the southern North Sea they provide evidence for both environmental and temporal varia tions. The cores located in the vicinity of the Pleistocene hinterland embrace the coastal development of the entire Holocene, reflecting "on shore" sedimentation conditions. A more or less "off shore" environment is indicated by Distribution and palaeoecological indicator value of diatoms 55 the sampling sites on the East Friesian Islands which mirror the youngest coastal develop ments of the North Sea only. Consequently, the results are initially presented separately and then brought together comprehensively.

Environmental interpretation of the borehole units of the Pleistocene hinterland
Basal peat, intercalated peat: Most basal peat units are characterized by a transition from carr to reed or sedge peat. An exception is Wanger land core W5 which records a change to bog peat .
The concentration of diatoms in the basal or ganic rich deposits varies rather widely. In the Loxstedt core, diatoms are absent in the lower most part of the basal peat. Only the freshwater mud ( Fig. 3) contains fragments of large Pinnu laria species (DELLWIG 1999) indicating limnic conditions. The upper part of the basal peat is characterized by the oligohalobous to freshwa ter species Staurosira construens. In addition, low numbers of strongly silicified polyhalobous pelagic diatoms, such as Triceratium favus and Aulacodiscus argus (not shown), were recorded. These indicate sea-water impact, possibly due to a single flood event. With regard to the dia tom assemblages it is obvious that the basal peat of the Loxstedt core developed under limnic conditions. By contrast the basal peat of the Wangerland core W2 (Fig. 4) received much more marine influence. Here the number of species is significantly higher than in the basal peat of the Loxstedt core, and brackish species, such as the mesohalobous Nitzschia constricta and the fragile N. sigma are dominant. In particular, N. sigma is an indicator of calm depositional conditions typical of the lower supratidal zone. Nevertheless, the presence of strongly silicified polyhalobous pelagic species like Paralia sulcata suggests flood events. Similarly the basal peat of the Schweiburg core ( Fig. 6) was also influenced by the North Sea. This is evident from the high numbers of strongly silicified allochthonous marine pelagic diatoms in the entire peat accu- mulation. The biostratigraphy of the basal peat units of the Wangerland and Schweiburg cores, comprising meso-and polyhalobous diatoms, indicate brackish lake conditions periodically invaded by sea water. These basal peats accu mulated under marine influence, whereas those of the Loxstedt core developed independent of marine influence and sea-level rise. The clastic coastal deposits of the Schweiburg, Loxstedt and Wangerland W2 and W5 cores also contain intercalated peat layers. Most of these organic layers consist of reed peat com prising species such as Phragmites australis which tolerates a salinity of about 10%o (SCHEER 1953). However, the occurrence of Phragmites peat suggests a slow-rising or even regressive sea level. An exception is the intercalated peat of the Schweiburg borehole which shows a transition to bog vegetation, reflecting a com plete progradation cycle. The topography of the natural levees in the ancient Jade Bay area may have prevented the direct inflow of seawater to this site (DELLWIG 1999). Due to marine influ ence, the intercalated peat layers are dominated by oligohalobous as well as marine pelagic dia toms.
The littoral species of the Wangerland core W5, Navicula capitata, Pinnularia viridis (not shown) and Anomoeoneis sphaerophora, indicate a salinity of 0-10%o (Fig. 5). At present all three species reside in the supratidal salt marshes and saline waters of the hinterland. Oligohalobous spe cies such as Diploneis interrupta are dominant in the intercalated peat layers of Wangerland core W2 (Fig. 4), and highly abundant in corre sponding layers of the Loxstedt core (DELLWIG 1999). In contrast, the intercalated peat layers of the Schweiburg core are characterized by the frequent occurrence of littoral polyhalobous and mesohalobous species with a wide salin ity range, e.g. Nitzschia navicularis and N. sigma. However, diatom assemblages in all intercalated peat layers were influenced by allochthonous marine pelagic species due to the proximate coastline and single flood events (e.g. Actinoptychus senarius and Paralia sulcata). In conclusion,  the composition of these assemblages indicates palaeoenvironmental conditions with a salinity of about 10%o.

Tidal channel deposits:
The Loxstedt borehole contains a tidal channel facies. In these deposits, the diatom record is poorly represented by a minor number of valves and taxa (Fig. 3). The assemblages are dominated by marine pelagic species, whilst the percentages of littoral diatoms are below 5%. High current velocities and low irradiance in tidal channels provide an unsuitable habitat for littoral species. The most prominent diatom species of the tidal channel deposits of the Loxstedt core is Paralia sulcata (Fig. 3). The high quantity of polyhalobous species recorded in these deposits suggests that salinity was close to 30%o, which is more or less equal to the salin ity level of the adjacent southern North Sea.
Tidal flat sediments: All cores contain tidal flat deposits. Samples from these sediments show diatom assemblages dominated by littoral species. Nevertheless, polyhalobous pelagic spe cies also commonly occur. The tidal flat sedi ments of the Wangerland core W2 are associ ated with a littoral diatom assemblage typical of muddy intertidal sediments, rich in species diversity (Fig. 4). The assemblage is dominated by polyhalobous and mesohalobous littoral spe cies such as Navicula digitoradiata, Nitzschia constricta and Nitzschia sigma. All these species can be found growing on mud in recent intertidal to lower supratidal areas with a salinity tolerance range of 5-30%o ( Table 2). Some of the more fragile species may be autochthonous. The oc currence of Navicula digitoradiata together with Diploneis interrupta, which today lives mainly in salt marshes, suggests a palaeoenvironmental level close the mean high water. In comparison the tidal flat sediments of Wangerland core W5 are dominated by strongly silicified polyhalobous species such as Delphineis surirella, with a salinity tolerance range of 20- 30%o (Fig. 5). The relatively high percentage of polyhalobous pelagic diatoms, such as Paralia sulcata, indicates transport conditions and a continuous marine influence. Fragile littoral species of meso-and oligohalobous tolerance ranges occur only in low quantity (> 5%). This suggests the palaeosalinity of the tidal flat was most likely higher in Wangerland core W5 (> 20%o) than in core W2 (< 20%o). Due to the position of core W2, the adjacent tidal channel had less influence on sediment accumulation. By comparison, the diatom assemblages from the tidal flat deposits of the Loxstedt core are entirely dissimilar to those of the Wangerland cores (Fig. 3) The tidal flat sequences of the Schweiburg (Fig. 6) and Wangerland W2 cores are characterized by the frequent occurrence of the littoral species Achnantbes delicatula. At present, this polyhalobous species lives on the sandy sediments of the lower intertidal zone, as well as in shallow subtidal areas, providing irradiance is sufficient for photosynthetic activity (Vos & DE WOLF 1988). The occurrence of epipsammic species, together with very low densities of mesoand oligohalobous epipelic species is typical for in tertidal sand flats. Due to the relatively high hydrodynamic energy, sedimentation is limited for both clay aggregates and pelagic diatoms. Therefore the relative abundance of these species is low (Vbs & DE WOLF 1994) and the littoral species predominate. Furthermore, the relatively low numbers of diatom valves and taxa in the tidal flat deposits of the Sch weiburg core confirm temporarily higher energy conditions. Fragile species, if once present, may not have survived or may be poorly represented due to subsequent reworking of sediments.

Brackish tidal flat:
In addition to tidal flat deposits with polyhalobous conditions, the ma jority of investigated cores also contain brackish deposits. A general increase of littoral species is clearly visible in these sediments. Among these, the number of polyhalobous diatoms decreases, while mesohalobous species increase. Further-Distribution and palaeoecological indicator value of diatoms 59 more, fragile forms show higher abundances. This suggests that the hydrodynamic energy was rather low in comparison to the polyhalobous tidal flat deposits mentioned above.
In the Loxstedt core, brackish deposits in the lower clastic unit are dominated by the littoral species Nitzschia sigma and N. brevissima (Fig. 3), with salinity tolerance ranges of 10-30%o and 0-20%o, respectively. The overlapping tolerance ranges of these species suggest saline conditions of < 20%o. This is supported by the occurrence of the fragile mesohalobous littoral species Gyrosigma spenceri (tolerance range 10-30%o). Fragile diatoms represent more than 40% of the total assemblage, thus suggesting calm depositional conditions. The species combina tion N. sigma and G. spenceri points to intertidal to lower supratidal mudflat conditions (Vos & DE WOLF 1988). The brackish deposits of the upper clastic unit of the Loxstedt core can be interpreted as a fluvially influenced brackish or even lagoonal palaeoenvironment of low salin ity, calm hydrodynamic conditions and clear water, a view that is supported by the presence of the oligohalobous epiphytic species Cocconeis placentula.
The two brackish layers of Wangerland core W5 are both characterized by high numbers of taxa. The brackish layer in the upper clastic unit contains several valves (Fig. 5). The dominant species of the lower brackish layer, Navicula radiosa (not shown), is sensitive to salinities higher than 20%o (PANKOW 1990). The upper brackish layer is characterized by the occurrence of meso halobous species, such as Nitzschia sigma, which indicate mud-enriched sediments close to or above the mean high water level and a salinity of ca 20%o (DELLWIG 1999).
Both are epipelic species that tolerate a wide salinity range (0-20%o). Today, they occur in the intertidal to lower supratidal zone. In the upper clastic unit of the W2 core, the brackish deposits are dominated by Tryblionella navicularis (not shown). This species also tolerates a wide salinity range (7-30%o). It occurs together with Diploneis smithii and D. interrupta, both of low abundance. In these brackish layers the portion of polyhalobous species such as Paralia sulcata, is higher than in the lower brackish layer, indicat ing a stronger marine influence. In summary, the brackish deposits are in general character ized by high values of mesohalobous littoral diatoms and those polyhalobous species which tolerate a wide range of salinities.

Brackish backside lagoons:
The lagoonal lay ers are also characterized by calm depositional conditions and a limited marine influence. Lit toral polyhalobous and mesohalobous species decrease by comparison to the brackish deposits, whereas oligohalobous littoral diatoms increase. It is noteworthy that in the Loxstedt and Sch weiburg cores, limnic diatoms also occur in the lagoonal deposits thereby indicating freshwater influence or episodic limnic conditions. The lower lagoonal section of the W5 core (Fig.  5) is dominated by Staurosira construens, which is characteristic of shallow lakes and ponds with a salinity <5%o (Vos & DE WOLF 1994). In the lagoonal facies of the W5 upper clastic unit, oligohalobous diatoms with salinity tolerance ranges of 0-10%o are frequent, e.g. Anomoeoneis sphaerophora and Navicula capitata.
In the lagoonal facies of the lower and upper clastic units of the Loxstedt core, epiphytic spe cies are frequent, e.g. the oligohalobous diatom Cocconeis placentula (Fig. 3). The presence of epiphytic diatoms points to a submerged en vironment with limited marine influence. The lagoonal layer in the middle part of the Loxstedt core is dominated by the littoral oligohalobous species Diploneis elliptica and D. interrupta. Both indicate restricted marine influence, which is typical for brackish water in the hinterland. The diatom assemblage of the lagoonal sedi-merits of Wangerland core W2 is similar to that of the lagoonal deposits in the Loxstedt core (Fig. 3, 4). Both layers are dominated by Diploneis species. Furthermore, Nitzschia sigma is frequent in both lagoonal deposits. Due to the dominance of Diploneis species, the palaeoenvironment may be interpreted as a muddy area in the supratidal zone with restricted marine influence. Generally, the lagoonal sediments are character ized by high values of fragile littoral species. The highest abundance of diatoms of all sedi ments in this study was found here.

Salt-marsh deposits:
The top of all investi gated cores consist of salt-marsh sediments. In these layers, the number of valves and taxa are rare in comparison to the lagoonal facies. Most salt-marsh deposits are dominated by alloch thonous marine pelagic species such as Paralia sulcata. Their occurrence may be related to single flooding events. In all cores, the predominant littoral species is Diploneis interrupta. This epi pelic species, which tolerates a salinity range of 0.2-10%o, currently lives in the brackish pools of salt marshes and hinterland areas which can periodically dry out.
Natural and fluvial levees: Natural levees mainly consist of sand accumulated in the su pratidal zone. These elevated levees, deposited during higher storm floods, are usually border ing the shoreline. However, fluvial levees with an average width of 1.5 km form a type of em bankment along the rivers Ems, Weser and Elbe (STREIF 1990). The Schweiburg and Wanger land W2 and W5 cores include natural levee sediments (Fig. 4-6), whereas only the Loxst edt core contains sediments of a fluvial levee from the river Weser (Fig. 3). These sediments are dominated by pelagic polyhalobous and oligohalobous pelagic species of allochthonous origin. The predominant diatom species in this section is Cyclotella meneghiniana which today is also the typical species of the river Weser. Marine pelagic diatoms, such as Paralia sulcata and Actinoptychus senarius, are intermixed and in dicate single flooding events. The high number of allochthonous diatoms outnumbers those of autochthonous origin suggesting that the palaeoenvironment was strongly influenced by transport.
In contrast to these fluvial levee deposits, the natural levee deposits of the Wangerland cores W2 and W5 are characterized by high numbers of valves and species (Fig. 4 -6). In particular, littoral species are prevailing, predominated by the epipelic littoral species Tryblionella navicularis and Nitzschia sigma, which tolerate a wide range of salinities. They may indicate areas behind a natural levee protected from marine influence. According to the diatom assemblages, a poorlydrained supratidal environment is assumed in which very shallow pools developed. Never theless, allochthonous pelagic species such as Paralia sulcata also occur in these sediments, thus reporting transport processes on the occasion of high tides and storm events.

Environmental interpretation of the profiles from the East Friesian Islands
Palaeoecological studies on the East Friesian Is lands were carried out for the reconstruction of former sea-levels. Salt-marsh horizons and tidal flat sediments are key sources of evidence as they provide material that is amenable to radio carbon dating and also offer the opportunity for palaeoenvironmental reconstruction by their content of macro-and microfossils (including diatoms, pollen, botanical and faunal remains). The hypothesis that diatom assemblages are related to different altitudes in salt-marsh veg etation, and the proof that diatom analysis on the basis of these observations is applicable to fossil records, was demonstrated by ZONG & HORTON (1999). Tidal flat sediments: Tidal flat sediments oc cur on Juist in most of the basal sections of the profiles. They show a distinct and typical succes sion pattern (Fig. 7). The assemblages are domi nated by frequently occurring littoral mesohalo bous species, with polyhalobous pelagic species such as Paralia sulcata and Actinoptychus senarius, recorded in relatively large numbers. The most abundant littoral species in the low-lying parts of the profiles is the epipelic and mesohalobous Scolioneis tumida. This species is typical of muddy inter-tidal sediments, but is currently also abun dant on sandy tidal flats in the vicinity of Juist (HUSTEDT 1939). The palaeoenvironment for this diatom assemblage was probably the lower part of the transition zone between intertidal and supratidal areas, with a palaeosalinity range of 5-30%o (Vos & DE WOLF 1988). There is a certain degree of variation in the succeeding zones of tidal flat sediments in the vicinity of Juist. S. tumida is no longer the dominant taxon, whereas species such as Navicula digitoradiata and Diploneis bombus occur more frequently and in high percentages. This association, combined with Tryblionella navicularis, Diploneis elliptica or the oligohalobous pelagic species Cyclotella meneghiniana is very similar to the assemblages described for the brackish tidal flats in the vicin ity of the Pleistocene hinterland in Wangerland. The latter species is very common in the water column of the river Ems and could therefore be distributed onto the backside tidal flat of Juist through the water current of the Osterems channel, passing the island on its western side. It is suggested that at that time sedimentation took place relatively close to the supratidal zone near the mean high tide level, also characterized by the typical mean high water taxon Navicula peregrina. The next diatom assemblage zone is most likely related to the same facies. However, that it relates closer to the supratidal zone is somewhat certain, because here littoral species occur together with Diploneis interrupta which today characterizes salt-marsh habitats. Another typical element of the upper inter-tidal zone, as well as of the lower salt-marsh areas on the East Friesian islands, is Caloneis formosa (syn. Caloneis westii, PANKOW 1990). The fossil diatom record of the Borkum tidal flat sediments shows some differences in con trast to the results of Juist (Fig. 8). This may be due to the geographical position of Borkum. Al though all East Friesian tidal flats are protected by the chain of barrier islands, the marine influ ence on Borkum, the biggest and most westerly lying East Friesian Wadden Sea island seems to be more distinct. Therefore it is understandable that the highest amounts of marine pelagic dia-  torn species such as Paralia sulcata and Podosira stelligera were recorded here. In addition, other polyhalobous species like Cymatosira belgica and Dimerogramma minor are probably representative of sandy mudflats (Fig. 8). Another typical spe cies of the tidal flat sediments of Borkum, with values of up to 60%, is Opephorapacifica (profile Borkum "Am großen Schlot", not presented in this paper). In contrast, this species was only re corded with very low values in the Juist samples. However, this polyhalobous species is at present one of the most common species in tidal flats of the North Sea. The low representation in the fossil diatom record from Juist is therefore unexpected.
A special feature of the tidal flat sequence of the profile "Am Hopp" is a distinct peak of Achnanthes delicatula, which occurs simultaneously with a decrease in percentages of Paralia sulcata. A lowering of sea level around AD 1287-1396; 660±60 BP) diminished marine influence and led to a localised and short-lived extension of a patchy and sparse plant cover with Salicornia, a conclusion supported by values of ca 50% for Salicornia type pollen (FREUND & STREIF, 1999). It is most likely that the vegetation cover functioned as a sand trap that provided suitable habitat for the epipsammic species Achnanthes delicatula on an intertidal sand flat. This hy pothesis is also supported by the fact that the frequencies of pelagic and/or epipelic taxa are relatively constant but generally lower through out this zone. The subsequent marine transgres sion phase shows an almost identical succession pattern as was demonstrated for the Juist tidal flat sequences, with an abundance of Diploneis bombus, Navicula digitoradiata and Caloneis formosa. The occurrence of N. cincta as a meso-oligohalobous species, and the associated silty sediment occurring at that time, suggests that sedimenta tion took place close to the supratidal area in shallow water and under calm conditions. The salinity was probably between 10-30%o.
Salt-marsh deposits: Although salt marshes look uniformly flat, there are clearly recogniz able differences in vegetation zonation. Under natural conditions this zonation reflects soil salinity and the frequency and duration of inundation. In contrast to the salt marshes in contact with the Pleistocene hinterland, the clastic matrix of salt-marsh soils on the East Friesian islands is relatively uniform and sandy. However, the halophytic plant communities covering these habitats are similar. The diatom record of the low salt-marsh de posits is dominated by Diploneis interrupta, an epipelic and mesohalobous species which pres ently lives in brackish pools of salt marshes ( Fig.  9 and 10). This taxon is often accompanied by Navicula peregrina, which normally coincides with clayey and silty sedimentation and reflects the calm conditions of sediment deposition in the low salt-marsh vegetation units. This is also supported by the occurrence of fragile, weakly silicified taxa such as Nitzschia sigma, giving this diatom assemblage a relative altitude around the local mean high water level. Most of these  (STREIF 1990). The ef fect of leaching rainwater is greater due to the diminished delivery of salt and therefore salinity is decreasing, reaching values of around 15%o. However, it has to be taken into account that the transition between lower and upper saltmarsh habitats is not sharp. Therefore, some of the low marsh and tidal flat species described above also occur in lower percentages in diatom assemblages of the upper salt marsh. A charac teristic feature of this zone is the occurrence in increasing frequencies of oligohalobous species such as Navicula pusilla, N. mutica, Pinnularia microstauron, P. borealis, P. major, and tiant^schia amphioxys. However, it must be noted that some freshwater diatoms such as N. pusilla tolerate short-term rises in water salinity up to 20%o (ZONG & HORTON 1997). Within this zone it is also evident that aerophilous taxa are becoming increasingly more important than in the lower altitudes of the salt marshes.
Dune slack deposits: Dune slack ecosystems are a special feature of Wadden Sea islands (PE TERSEN 2000). In general, dune slack vegetation is distributed in valleys between the west-east stretching dune ridges. The valley bottoms are flooded mainly during winter due to influence from the groundwater table. As a result, peat deposits such as brown moss peat often accu mulate. Unfortunately dune slack deposits are very rare, so that only one profile could demon strate the succession pattern (profile Langeoog 3, Fig. 11).
In the basal layers of the profile, Diploneis inter rupta is recorded as the dominant taxon whereas other species occur only in traces. As preserva-tion was very poor, a high degree of erosive selection has to be taken into account making this zone unsuitable for ecological interpreta tion. The subsequent zone is associated with terrestrialization or the beginning of peat ac cumulation. The diatom assemblage is initially dominated by Epithemia %ebra var. porcellus, an epiphytic species which lives in shallow water and tolerates salinities up to 10%o. Littoral, brackish mesohalobous species such as Navicula peregrina are frequent and document a continu ous but minor marine influence. However, oligohalobous and aerophilous species are the main component, e.g. Pinnularia div. spec, Cymbella aspera, Diploneis elliptica, and Navicula mutica. The deposit is a sedge peat consisting of numer ous root fragments of undeterminable sedges. Following this zone there is a transition from sedge peat, with few brown moss remains, to a peat deposit mainly consisting of various moss species like Drepanocladus exannulatus, D. fluitans, and Calliergon cordifolium. This change is also reflected in the diatom assemblage. Pin nularia gibba, P. major and Navicula variostriata are increasing simultaneously, arguably suggesting oligohalobous conditions in a swampy envi ronment. However, there is a short-term, weak increase in salinity, reflected by a sharp increase in numbers of Staurosira construens var. subsalina, a species which lives in shallow, moderately sa line ponds. This change and the presence of the subsequent sand layer reflect the unfavourable environmental conditions leading to sand drift and severe inundations during the 17 th century, with the consequence that Langeoog was briefly inhabited during that period. The subsequent peat deposit, which possibly due to protection from sand dunes accumulated independently of severe marine influences, is again dominated by different Pinnularia species.

Diatom groups and their application to the reconstruction of palaeoenvironments
It is evident from the diatom diagrams that some assemblages occur repeatedly with the same succession pattern thereby characterizing a distinct palaeoenvironment. Therefore it is necessary to classify the recorded diatom assem blages into different ecological groups, which could be used for the determination of coastal palaeoenvironments in the southern North Sea (Table 2). However, as there is no direct correlation be tween those groups and tidal levels, an indirect correlation has to be assumed. It is apparent that the habitat of every diatom is related to the hy drodynamic regime as well as the frequency of inundation. Correspondingly, species are differ entiated according to their preference zones at sub-, inter-or supratidal levels (Vos & DE WOLF 1988). The fragility of diatom frustules has of ten been used to estimate the importance of palaeotransport. Fragile species in the sequences indicate that they have lived at the site of depo sition, and thus characterize the palaeoenviron ment more effectively than the strongly silicified species. Generally, the influx of allochthonous species in low-energy environments can be ignored (Vos & DE WOLF 1988). A suitable cri terion for the importance of transport processes is the ratio between pelagic and littoral diatoms. A dominance of pelagic diatoms, which live mainly in the open sea, indicates that transport processes prevailed, whereas littoral species were deposited predominantly in or adjacent to their biotopes (SIMONSEN 1962). High ratios of ma rine pelagic species in limnic or oligohalobous environments verify a neighbouring shoreline and periodical marine influence. Therefore, it is important to differentiate between oligo halobous and polyhalobous pelagic diatoms. Salinity is probably the strongest environmental factor influencing the distribution of diatoms (BATTARBEE 1986). Consequently, diatoms are Distribution and palaeoecological indicator value of diatoms 67 good indicators of long-term salinity changes in coastal areas caused by sea level fluctuations and marine influence. All of the above criteria influence species com position and must therefore be considered in the reconstruction of palaeoenvironments in Holocene coastal deposits.

Holocene diatom records as indicators of anthropogenic influences on coastal environments
The recent coastal area of the southern North Sea has been formed by mainly natural proc ess over the last 8000 years. Human activities, commencing around 2000 years BP with the building of settlements in the Roman Period and Early Middle Ages, had minor impact un til the 11 th century when the building of dikes created a distinct interaction with hydrological and climatically derived physical processes. One of the consequences was a change in the hydrographic conditions of the inundated area, i.e. the restriction of the supratidal zone. There fore, the assemblages and distribution patterns of diatoms were affected due to the versatility of habitats. In addition, the swamp-type char acteristics of the supratidal zone also changed. The present-day salt marshes are well drained because of land reclamation and pasture. The modern supratidal zone is relatively close to tidal channels and experiences marine influence (ANDERSON & Vos 1992). Diatom analyses in this study suggest the earlier salt marshes and the broad hinterland zone were increas ingly swampy at this time. In all cores of the Pleistocene hinterland, the supratidal layers are characterized by larger numbers of diatoms with wider diversity (DELLWIG 1999) than in present-day salt marshes. The species Diploneis interrupta reached a particularly high abundance in the salt marshes of the mainland as well as the East Friesian islands. This species is nowadays very rare in samples of anthropogenically-influenced environments . Dikes also result in the reflection of waves which in former times lost their energy as they dispersed over a wide hinterland. These changes have contributed to a gradual increase of coars er sediments in the vicinity of the shoreline, whilst muddy sediments steadily become rarer (FLEMMING & DAVIS 1994). Such a change of the sedimentary character of the tidal flats from muddy to coarser sediments has effected the diatom populations adapted to different grain-size compositions. Therefore the total number of epipelic species decreases, whilst the epipsammic diatoms increase. Changes in littoral diatom populations also have important implications for the food-web of shallow coastal ecosystems, in which large numbers of filter and deposit-feeders primarily feed on littoral diatom cells (DE JONGE 1992). The quantitative and qualitative reduction in diatoms will not only affect the tidal ecosystems, but also has im plications for the North Sea in general because of the important role of the tidal ecosystems as nursery and breeding grounds for offshore com munities (ANDERSON & VOS 1992). Although this study is limited to four cores on the mainland and five profiles of the East Friesian Islands, it illustrates the effectiveness of small scale facies analysis as a tool in coastal dia tom palaeoecology. Knowledge of the changing palaeoenvironmental conditions is especially relevant for understanding anthropogenicallyinduced changes in the coastal areas. Fossil diatom assemblages are useful natural indicators providing palaeoecological data for the assess ment of recent environmental changes.